Congestion_control_Open_Loop_and_closed_loop_Choke_Packets_PPT.pptx

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1. Course name - NETWORK PROTOCOLS & SECURITY Course Code - 21EC2210 Session - 25 Topic: Congestion Control: OPEN LOOP and Closed Loop Choke Packets DEPARTMENT OF CSE

2. AIM OF THE SESSION To familiarize students with the basic concept of Congestion control and its categories INSTRUCTIONAL OBJECTIVES This Session is designed to: Introduction to Data traffic and Congestion control techniques Various categories of Congestion control mechanism with diagram LEARNING OUTCOMES At the end of this session, you should be able to: Describe the concepts of network performance Describe the components of the Congestion control mechanism List out Various categories of Congestion prevention and removal mechanism

3. AGENDA Discussion about Data Traffic in Congestion Various key Traffic descriptors and Three traffic profiles About congestion, Congestion Queue Policies and Network Performance Two congestion control mechanisms Policies adopted by open & closed loop congestion control Purpose of Congestion control Summary Self-Assessment Q uestions Terminal questions References

4. Data Traffic The main focus of congestion control and quality of service is data traffic . In congestion control we try to avoid traffic congestion . In quality of service, we try to create an appropriate environment for the traffic. So, before talking about congestion control and quality of service, we discuss the data traffic itself. Congestion Control

5. Traffic descriptors Average Data Rate Average data rate = amount of data / time The average data rate is a very useful characteristic of traffic because it indicates the average bandwidth needed by the traffic Peak Data Rate The peak data rate defines the maximum data rate of the traffic. The peak data rate is a very important measurement because it indicates the peak bandwidth that the network needs for traffic to pass through without changing its data flow .

6. Traffic descriptors   Maximum Burst Size Although the peak data rate is a critical value for the network, it can usually be ignored if the duration of the peak value is very short. The maximum burst size normally refers to the maximum length of time the traffic is generated at the peak rate .   Effective Bandwidth The effective bandwidth is the bandwidth that the network needs to allocate for the flow of traffic. The effective bandwidth is a function of three values: average data rate, peak data rate, and maximum burst size. The calculation of this value is very complex.

7. Three traffic profiles Constant Bit Rate A constant-bit-rate (CBR), or a fixed-rate, traffic model has a data rate that does not change. In this type of flow, the average data rate and the peak data rate are the same. The network knows in advance how much bandwidth to allocate for this type of flow. Bursty In the bursty data category, the data rate changes suddenly in a very short time. It may jump from zero, for example, to 1 Mbps in a few microseconds and vice versa. Bursty traffic is one of the main causes of congestion in a network. Variable Bit Rate In the variable-bit-rate (VBR) category, the rate of the data flow changes in time, with the changes smooth instead of sudden and sharp. In this type of flow, the average data rate and the peak data rate are different.

8. Congestion An important issue in a packet-switched network is congestion. Congestion in a network may occur if the load on the network-the number of packets sent to the network-is greater than the capacity of the network-the number of packets a network can handle. Congestion control refers to the mechanisms and techniques to control the congestion and keep the load below the capacity. We may ask why there is congestion on a network. Congestion happens in any system that involves waiting. For example, congestion happens on a freeway because any abnormality in the flow, such as an accident during rush hour, creates blockage Continue.,.

9. Queues in a router Congestion in a network or internetwork occurs because routers and switches have queues-buffers that hold the packets before and after processing . A router, for example, has an input queue and an output queue for each interface. When a packet arrives at the incoming interface, it undergoes three steps before departing, as shown in Figure Continue.,.

10. The packet is put at the end of the input queue while waiting to be checked. The processing module of the router removes the packet from the input queue once it reaches the front of the queue and uses its routing table and the destination address to find the route. The packet is put in the appropriate output queue and waits its tum to be sent. We need to be aware of two issues . First , if the rate of packet arrival is higher than the packet processing rate, the input queues become longer and longer. Second , if the packet departure rate is less than the packet processing rate, the output queues become longer and longer. Queues policy

11. Congestion control involves two factors that measure the performance of a network: delay and throughput . Figure shows these two performance measures as function of load. Network Performance

12. Congestion Control mechanisms Congestion control refers to techniques and mechanisms that can either prevent congestion, before it happens, or remove congestion, after it has happened. In general, we can divide congestion control mechanisms into two broad categories: open-loop congestion control and closed-loop congestion control.

13. Congestion control categories Policies adopted by open & closed loop congestion control

14. Open loop congestion control policies are applied to prevent congestion before it happens. The congestion control is handled either by the source or the destination. Open Loop Congestion Control Retransmission Policy If the sender feels that a sent packet is lost or corrupted, the packet needs to be retransmitted. This transmission may increase the congestion in the network.  To prevent congestion, retransmission timers must be designed to prevent congestion and also able to optimize efficiency.  Window Policy The type of window at the sender’s side may also affect the congestion . Several packets in the Go-back-n window are re-sent, although some packets may be received successfully at the receiver side. This duplication may increase the congestion in the network and make it worse. Therefore, Selective repeat window should be adopted as it sends the specific packet that may have been lost.

15. Discarding Policy A good discarding policy adopted by the routers is that the routers may prevent congestion and at the same time partially discard the corrupted or less sensitive packages and also be able to maintain the quality of a message . In case of audio file transmission, routers can discard less sensitive packets to prevent congestion and also maintain the quality of the audio file. Acknowledgment Policy Since acknowledgements are also the part of the load in the network, the acknowledgment policy imposed by the receiver may also affect congestion. Several approaches can be used to prevent congestion related to acknowledgment. The receiver should send acknowledgement for N packets rather than sending acknowledgement for a single packet. The receiver should send an acknowledgment only if it has to send a packet or a timer expires

16. Admission Policy In admission policy a mechanism should be used to prevent congestion. Switches in a flow should first check the resource requirement of a network flow before transmitting it further. If there is a chance of a congestion or there is a congestion in the network, router should deny establishing a virtual network connection to prevent further congestion All the above policies are adopted to prevent congestion before it happens in the network

17. Congestion control categories Policies adopted by open & closed loop congestion control

18. Closed loop congestion control techniques are used to treat or alleviate congestion after it happens. Several techniques are used by different protocols; some of them are:  Closed Loop Congestion Control Backpressure The technique of backpressure refers to a congestion control mechanism in which a congested node stops receiving data from the immediate upstream node or nodes. This may cause the upstream node or nodes to become congested, and they, in turn, reject data from their upstream nodes or nodes. And so on. Backpressure is a node-to-node congestion control that starts with a node and propagates, in the opposite direction of data flow, to the source. Continue.,.

19. Closed loop congestion control techniques are used to treat or alleviate congestion after it happens. Several techniques are used by different protocols; some of them are:  Closed Loop Congestion Control Backpressure The backpressure technique can be applied only to virtual circuit networks , in which each node knows the upstream node from which a flow of data is corning . Below Figure shows the idea of backpressure.

20. Choke packet technique is applicable to both virtual networks as well as datagram subnets. A choke packet is a packet sent by a node to the source to inform it of congestion. Each router monitors its resources and the utilization at each of its output lines. Whenever the resource utilization exceeds the threshold value which is set by the administrator, the router directly sends a choke packet to the source giving it a feedback to reduce the traffic. The intermediate nodes through which the packets has traveled are not warned about congestion.  Choke Packet Technique

21. In implicit signaling , there is no communication between the congested nodes and the source. The source guesses that there is congestion in a network. For example when sender sends several packets and there is no acknowledgment for a while, one assumption is that there is a congestion.  Implicit Signaling

22. Explicit Signaling The node that experiences congestion can explicitly send a signal to the source or destination . The explicit signaling method , however, is different from the choke packet method . In the choke packet method, a separate packet is used for this purpose; in the explicit signaling method, the signal is included in the packets that carry data. Explicit signaling , as we will see in Frame Relay congestion control, can occur in either the forward or the backward direction.

23. Explicit Signaling Backward Signaling: A bit can be set in a packet moving in the direction opposite to the congestion. This bit can warn the source that there is congestion and that it needs to slow down to avoid the discarding of packets. Forward Signaling: A bit can be set in a packet moving in the direction of the congestion . This bit can warn the destination that there is congestion. The receiver in this case can use policies, such as slowing down the acknowledgments, to alleviate the congestion.

24. The purpose of Congestion Control is a mechanism that controls the entry of data packets into the network, enabling a better use of a shared network infrastructure and avoiding congestive collapse Purpose of Congestion Control mechanism

25. SUMMARY Congestion control techniques and mechanisms Various categories of Congestion control for prevention and removal mechanism with diagram

26. SELF-ASSESSMENT QUESTIONS X=301, Since packets with sequence (301-400) is lost After 3 duplicate acknowledgements (mild congestion) -(301-400) will be send again . Y=301 Z=400 X+Y-Z=301+301-400=602-400=202 Why does congestion occur? Backward Explicit Congestion Notification (BECN ) In Frame relay, the _____________bit warns the sender of congestion in the network Because the routers and switches have queues In_______, queuing packets wait in a buffer(queue) until the node(router or switch) is ready to process them. FIFO Priority Weighted fair None of the above

27. TERMINAL QUESTIONS How are congestion control and quality of service related? What is a traffic descriptor? What is the relationship between the average data rate and the peak data rate? What is the definition of bursty data? What is the difference between open-loop congestion control and closed-loop congestion control? Name the policies that can prevent congestion. Name the mechanisms that can alleviate congestion .

28. REFERENCES FOR FURTHER LEARNING OF THE SESSION Text Books 1 Data Communications and Networking (3rd Ed.) “–B. A. Ferouzan – TMH 2 Computer Networks (4th Ed.)”, A. S. Tanenbaum – – Pearson Education/PHI Reference Text books 1 Data and Computer Communications (5th Ed.)” – W. Stallings – PHI/ Pearson Education 2 Network for Computer Scientists & Engineers, Zheng & Akhtar, OUP 3 Data & Computer Communication, Black, PHI 4 Data Communication & Network, Miller, Vikas 5 Digital & Data Communication, Miller, Jaico 6 Understanding Data Communication & Network, Shay Vikas Web References 1 Kurose and Rose – “Computer Networking -A top down approach featuring the internet”– Pearson Education 2 “Communication Networks” – Leon, Garica, Widjaja TMH 3 “Internetworking with TCP/IP, Comer – vol. 1, 2, 3(4th Ed.)” – Pearson Education/PHI 4 https://www.tutorialspoint.com/what-is-closed-loop-congestion-control/ 5 https://www.geeksforgeeks.org/congestion-control-techniques-in-computer-networks/3.

29. THANK YOU Team – NETWORK PROTOCOLS & SECURITY